1. Field of Invention
The present invention relates to a laser diode driving circuit for supplying a drive current to a laser diode, a semiconductor integrated circuit for driving the laser diode, and an image recording apparatus in which a step of executing a scanning on a predetermined member to be scanned with a laser beam holding image information is included in an image recording step.
2. Description of Related Art
A laser diode is conventionally used as a means for generating a light beam in an image recording device.
FIG. 8 is a diagram showing the light output characteristic of a laser diode. The axis of abscissa denotes currents supplied to the laser diode and the axis of ordinate shows a light output power for the current supplied.
As shown in the diagram, the light output characteristic of the laser diode is divided into an LED region when a current supplied to the laser diode is less than a predetermined light emission threshold current I.sub.th and a laser beam emission region when the supplied current is equal to or larger than the light emission threshold current I.sub.th. In the LED region, light is not substantially emitted from the laser diode. In the laser beam emission region, the light output of the power almost proportional to a drive current obtained by subtracting the light emission threshold current I.sub.th from the supply current can be obtained. Consequently, when the laser diode is driven so as to repeatedly emit lights and stop the emission at high speed, it is necessary to always flow a bias current which is almost equal to the light emission threshold current I.sub.th to the laser diode even at the time of no light emission.
As methods of obtaining half-tone by an image recording apparatus using the laser diode, there are a pulse width modulating method and a luminous intensity modulating method. The pulse width modulating method changes the exposure per pixel by changing the pulse width in accordance with image information. The luminous intensity modulating method changes the exposure per pixel by changing the power of the laser in accordance with image information. In the case of the pulse width modulating method, the processing speed of allowing the laser diode to emit light and to stop the emission is limited. When the high processing speed is intended to be achieved, either the luminous intensity modulating method or a modulating method in which the pulse width modulating method is combined with the luminous intensity modulating method can be considered.
FIG. 9 is a schematic diagram mainly showing a laser diode driving circuit part of a conventional image recording apparatus using the luminous intensity modulating method (refer to Published Unexamined Japanese Patent Application No. Hei 6-334248).
From a signal generating circuit 101, a digital image signal is outputted synchronously with a timing signal generated by a timing signal generating circuit 102, passed through a digital adder 103, converted by a D/A converter 104 into an analog image signal, passed through an aenalog adder 105, and amplified by a drive amplifying circuit 106 to drive a laser diode 107, and a current is supplied to the laser diode 107. As described with reference to FIG. 8, the laser beam of the light quantity proportional to the current portion exceeding the threshold current I.sub.th in the current supplied to the laser diode 107 is emitted. The laser beam emitted from the laser diode 107 is reflected and deflected by a polygon mirror 108 and a photosensitive member 109 on which an electrostatic latent image is formed is scanned by the irradiation of the laser beam, thereby forming an electrostatic latent image on the photosensitive member 109. The electrostatic latent image formed on the photosensitive member 109 is developed by toner in accordance with a known electrophotographic process and the toner image is transferred and fixed on a sheet.
The laser beam reflected and deflected by the polygon mirror 108 is detected by a photodetector 110 at scan start timings and the scan start timings are notified to the timing signal generating circuit 102. The timing generating circuit 102 receives the scan start timing signals and controls all of an output timing of the digital image signal outputted from the signal generating circuit 101, timings to read the maximum and minimum from a maximum and minimum generating memory 113, and sample and hold timings of two sample and hold circuits 114 and 115.
The maximum and minimum image signals are stored in the maximum and minimum generating memory 113 and are sequentially read out at timings when the laser beam emitted from the laser diode 107 and reflected and deflected by the polygon mirror 108 is deviated from the photosensitive member 109 and when there is no signal generated from the signal generating circuit 101 (the value "0" is outputted). The signals are supplied to the digital adder 103, D/A converter 104, analog adder 105, and drive amplifying circuit 106, and the currents corresponding to the maximum and minimum are supplied to the laser diode 107.
A part of the laser beam emitted from the laser diode 107 is received by a photodiode 111 and is converted into a voltage signal by an I/V converter 112 for converting current signals to voltage signals, and the resultant data is transmitted to the two sample and hold circuits 114 and 115. On the basis of the timing signals from the timing signal generating circuit 102, the sample and hold circuits 114 and 115 sample and hold the signals supplied from the I/V converter 112 at time points when the laser beams of the light quantities corresponding to the maximum and minimum read out from the maximum and minimum generating memory 113 are emitted from the laser diode 107, respectively, and supply the signals to comparing circuits 116 and 117. The comparing circuit 116 compares the supplied sampled and held signal corresponding to the maximum with a predetermined maximum signal Pmax, supplies the resultant data as a reference voltage of the D/A converter 104 to the D/A converter 104, and the scale of the D/A conversion is adjusted on the basis of the data. The comparing circuit 117 compares the sampled and held signal corresponding to the minimum with a predetermined minimum signal Pmin, supplies the resultant data as a bias voltage to the analog adder 105, and the bias of the whole signals supplied to the drive amplifying circuit 106 is adjusted on the basis of the data.
In the conventional technique shown in FIG. 9, the laser diode 107 is driven by the current signals in which the scale and the bias are adjusted and the luminous intensity is modulated as mentioned above.
According to the conventional technique, however, the current signal at the minimum level corresponding to the bias current is the minimum value in the signal level at which the latent image is formed on the photosensitive member 109, in other words, it is a minimum value Imin determined in the laser beam emission region shown in FIG. 8. When the current of the minimum value Imin is supplied to the laser diode 107, there is the possibility that, although it is weak, the laser diode 107 emits a light, so that an offset or fog occurs in the latent image formed on the photosensitive member 109, and a preferable reproduction image cannot be obtained.